Vero cells treated with 100 mg/ml of ribarivin (1-B-D-ribofuranosyl-1, 2, 4, triazole - 3 - Carboxamide) and infected with Halle, McClellan or Mantooth strains of measles virus yielded 100- to 1000-fold less infectious virus. The observed antiviral effect of ribavirin could be reversed by the addition of adenosine or guanosine. Ribovirin did not markedly reduce protein synthesis in measles virus infected cells. However, synthesis of virus-specific proteins was impaired, since virus specific fluorescent antibody and hemadsorption activity were diminished or abolished in cells treated with 400 mg/ml of the drug. The antiviral activity of ribavirin was reversible and not viracidal. After removal of ribavirin from measles virus-infected cells, virus replication continued. This finding suggests that viral RNA needed for transcription or translation is not degraded or altered in the presence of the drug.